The communication system that is a successor to WCDMA and HSDPA i.e. the LTE (Long Term Evolution) has been studied by 3GPP that is the standardization group of WCDMA, and as a radio access scheme, OFDMA (Orthogonal Frequency Division Multiplexing Access) is defined in downlink, while SC-FDMA (Single-Carrier Frequency Division Multiple Access) is defined in uplink (for example, see Non-patent Document 1).
OFDMA is a system for dividing a frequency band into a plurality of narrow frequency bands (subcarriers), and assigning data onto each frequency band to perform transmission. In OFDMA, subcarriers are arranged densely in the frequency domain without interfering with one another even with part thereof overlapping, high-speed transmission is thereby achieved, and spectral efficiency is enhanced.
SC-FDMA is a transmission system for dividing the frequency band so that a plurality of terminals uses different frequency bands to perform transmission, and thereby enabling interference among the terminals to be reduced. SC-FDMA has the feature that the fluctuation in the transmission power is decreased, and therefore, actualizes low power consumption in the terminal and wide coverage.
The LTE system is a system in which a plurality of mobile stations shares one, or two or more physical channels to perform communications both in uplink and downlink. The channel shared by a plurality of mobile stations is generally referred to as the shared channel, and in the LTE system, is the “Physical Uplink Shared Channel (PUSCH)” in uplink, while being the “Physical Downlink Shared Channel (PDSCH)” in downlink.
Further, as a transport channel, the shared channels are the “Uplink Shared Channel (UL-SCH)” in uplink, and the “Downlink Shared Channel (DL-SCH)” in downlink.
Then, in the communication system using the above-mentioned shared channels, it is necessary to select a mobile station to assign a shared channel for each sub-frame (1 ms in the LTE system), and perform signaling to notify the selected mobile station of the assignment of the shared channel.
The control channel used for this signaling is referred to as the “Physical Downlink Control Channel (PDCCH)” or “Downlink L1/L2 Control Channel (DL L1/L2 Control Channel)” in the LTE system.
In addition, the processing for selecting a mobile station to assign a shared channel for each sub-frame is generally referred to as “scheduling”. In this case, a mobile station to assign the shared channel is dynamically selected for each sub-frame, and the processing may be referred to as “dynamic scheduling”. Further, “assigning a shared channel” may be expressed by “assigning radio resources for the shared channel”.
For example, the information of the Physical Downlink Control Channel includes “downlink scheduling information”, “uplink scheduling grant”, etc. (for example, see Non-patent Document 2).
In addition, in LTE, discontinuous reception (DRX) control is applied. The discontinuous reception control is applied in the case that the base station apparatus and the mobile station are connected with each other and that data to communicate does not exit. The mobile station in the discontinuous reception state receives the Physical Downlink Control Channel (PDCCH) periodically i.e. intermittently. In this case, it is essential only that the mobile station receives the Physical Downlink Control Channel intermittently instead of all the timing, and it is thereby possible to reduce power consumption in the battery (battery saving). The time duration to intermittently receive the Physical Downlink Control Channel in the above-mentioned discontinuous reception control is referred to as ON duration of DRX or On-duration. Further, a cycle to set the On-duration is referred to as the DRX cycle.
Further, in uplink of LTE, timing synchronization is maintained on a signal transmitted from each mobile station in the cell. In other words, in uplink of LTE, the reception timing in the radio base station of the signal transmitted from the each user equipment in the cell is controlled to coincide with one another in a predetermined range. The above-mentioned control is referred to as Transmission Timing Adjustments (Non-patent Document 3).
More specifically, the radio base station measures the reception timing of a signal transmitted from the mobile station, and when the reception timing deviates from the reference timing defined inside the radio base station, transmits a control signal, Timing Advance, to correct the deviation to the mobile station in downlink. Then, based on the Timing Advance, the mobile station adjusts the transmission timing of an uplink signal.
In addition, the mobile station starts or restarts a Time Alignment Timer from the timing at which Timing Advance is received, and at the time the Time Alignment Timer expires, determines that uplink timing synchronization is not maintained. In the case that uplink timing synchronization is not maintained and that it is necessary to perform uplink transmission, the mobile station performs a random access procedure, and establishes uplink timing synchronization (Non-patent Document 4).
In addition, in the discontinuous reception state, generally, data exchange is not performed between the radio base station and the mobile station, and therefore, Timing Advance is not transmitted. In other words, in the discontinuous reception state, there are many cases that uplink timing synchronization is not maintained.
Then, in general, an operator in the mobile communication system offers mobile communication services to users, and collects charges as compensation. Hereinafter, the action of “collecting charges as compensation for offered mobile communication services” is referred to as charging. As a manner for charging, for example, there is charging in accordance with a data amount during communications, charging in accordance with communication time, etc. Alternately, as a flat-rate system, there is the case that the fixed fee is charged for a predetermined period of time e.g. one month irrespective of the data amount and communication time.